Borrelia burgdorferi, the causative agent of Lyme disease, requires shifts in gene expression to undergo its natural enzootic cycle between tick and vertebrate hosts. mRNA degradation is an important mechanism for gene regulation, but is not yet characterized in B. burgdorferi. We propose to characterize the roles of specific ribonucleases in mRNA turnover by comparing the decay rates and steady state abundances of transcripts in wild-type and ribonuclease deficient strains of B. burgdorferi. We will determine the consensus sequences and structures of mRNA recognized and cleaved by ribonucleases. In order to gain insight into the relationship between translational regulation and mRNA decay for genes important in the enzootic cycle, we will examine the rates of mRNA decay of transcripts in the presence and absence of their translational regulators. The data from the proposed experiments will provide researchers with , will be the first to provide insight into the roles of individual ribonucleases, characteristics of ribonuclease cleavage sites, and relationships between translational regulation and mRNA decay for selected genes in this important pathogen. Lyme disease infects approximately 300,000 U.S. citizens each year. A better understanding of the molecular mechanisms of gene regulation, including those involving mRNA decay, that are used by this pathogen to survive the transition between vector and host is essential for the design of improved diagnostic and treatment strategies.